Thank you very much for the pointer! This one has version 1.01. The OSMF source code for parsing F4M is talking about features introduced in F4M 2.x (e.g., the "alternate" attribute of the <media> element). That's why I hope to find the latest F4M specification. Or is it enough to simply support F4M 1.01 in practice?

The latest (2.0) F4M spec has not yet been published publicly, though it is due to be released within the next two weeks or so. Whether or not you need to comply with the new F4M 2.0 spec depends on what features you want to work with, DVR Rolling Window being the most prominent one. OSMF 1.6-based players can certainly still handle both 1.0 F4M files and 2.0 F4M files, though.

If you can hold on 'til tomorrow, I'll see if we're ready to push the spec out sooner rather than later. No promises, though, as I'm just the writer. :-)